Hello,

The following patches introduce a simple cache mechanism for both
`org-element-at-point' and `org-element-context'. My goal is to make
them fast enough to be used in most core commands (excepted
headlines-only commands).

Since a wrong cache can break Org behaviour badly, I would appreciate if
it could be tested a bit. You can disable cache at any time by setting
`org-element-use-cache' to nil and reset it with
`org-element-cache-reset' function.

It may also be interesting to tweak `org-element--cache-sync-idle-time'
and `org-element--cache-merge-changes-threshold', although I don't
expect a regular user to do it. Anyway, it may lead to better default
values.

Since cache is updated upon buffer modification, visibility status
cannot be cached properly. Since it is also buggy, the first patch
removes that data altogether.

Feedback welcome.


Regards,

-- 
Nicolas Goaziou